Uniform field means a constant field at every point and it is directed in a fixed direction for all points. Thus equipotential surfaces must be plane surface. The electric field is always perpendicular to equipotential surface because →E=−→∇V. As surfaces are equipotential so it must be spaced such that surfaces having equal differences in potential are separated by equal distances. As E=−dVdr so it must have decreasing potentials in the direction of the field.